fixed incorrect null object test in SceneWriter
fixed incorrect null object test in SceneWriter

boberfly/gaffer,cedriclaunay/gaffer,appleseedhq/gaffer,GafferHQ/gaffer,chippey/gaffer,goddardl/gaffer,davidsminor/gaffer,andrewkaufman/gaffer,ivanimanishi/gaffer,goddardl/gaffer,cedriclaunay/gaffer,ivanimanishi/gaffer,ImageEngine/gaffer,paulondc/gaffer,ivanimanishi/gaffer,ImageEngine/gaffer,davidsminor/gaffer,johnhaddon/gaffer,ivanimanishi/gaffer,hradec/gaffer,johnhaddon/gaffer,paulondc/gaffer,ImageEngine/gaffer,hradec/gaffer,GafferHQ/gaffer,appleseedhq/gaffer,andrewkaufman/gaffer,appleseedhq/gaffer,goddardl/gaffer,ImageEngine/gaffer,lucienfostier/gaffer,paulondc/gaffer,hradec/gaffer,boberfly/gaffer,andrewkaufman/gaffer,boberfly/gaffer,johnhaddon/gaffer,chippey/gaffer,johnhaddon/gaffer,hradec/gaffer,cedriclaunay/gaffer,johnhaddon/gaffer,andrewkaufman/gaffer,chippey/gaffer,paulondc/gaffer,lucienfostier/gaffer,lucienfostier/gaffer,andrewkaufman/gaffer,appleseedhq/gaffer,davidsminor/gaffer,boberfly/gaffer,lucienfostier/gaffer,GafferHQ/gaffer,cedriclaunay/gaffer,chippey/gaffer,hradec/gaffer,GafferHQ/gaffer,davidsminor/gaffer,goddardl/gaffer,GafferHQ/gaffer